forcedeth: fix checksum flag

Fix the checksum feature advertised in device flags.  The hardware support
TCP/UDP over IPv4 and TCP/UDP over IPv6 (without IPv6 extension headers).
However, the kernel feature flags do not distinguish IPv6 with/without
extension headers.

Therefore, the driver needs to use NETIF_F_IP_CSUM instead of
NETIF_F_HW_CSUM since the latter includes all IPv6 packets.

A future patch can be created to check for extension headers and perform
software checksum calculation.

Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Cc: Jeff Garzik <jgarzik@pobox.com>
Cc: Manfred Spraul <manfred@colorfullife.com
Cc: <stable@kernel.org>		[2.6.25.x, 2.6.26.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>